After inserting your groups, you should be using one formula per fiscal year. you would modify your formula above to look like:
if {SA_1_NF.Invoice_Date} in Cdate(2001,11,01)to CDate(2002,10,31) and
{SA_1_NF.Ord_Class} in ["AG","BVF","CA","CP","ER","ES","EU","OM","RM"] then {SA_1_NF.Sales}
Then you would place this formula in the detail section, right click on it and insert a summary at all group levels, and at the grand total level, if you like. Ordinarily, if you want a conditional summary, the intent is to apply the condition to detail records and then summarize the results.
If instead you place the summary within the formula itself and then place that formula in the group footer, you would only be getting the group summary for all records in the group (regardless of meeting the criteria) if the last detail record met the conditional criteria, and if it didn't, you would get 0.
-LB